The invention relates to free-flowing, chemically resistant colour granulates which are manufactured by build-up granulation, and their use.
Many pigments and inorganic colouring bodies which are used for colouring ceramics, enamels, glass, synthetic materials and so on, exhibit only a limited thermal or chemical stability. For example, enamel molten masses during processing are especially aggressive to inorganic colouring bodies because of their high B.sub.2 O.sub.3 or alkali contents. This brings, in the processing of many pigments in enamels to undesired colour shift or to the complete dissolution of the pigment particles having usually a size of about 0.2 to 0.7 microns.
Some colouring bodies are actually rather stable in enamel or ceramic molten masses, but contain environmentally questionable elements, such as lead, cadmium, five-valent antimony, six-valent chromium and so on.
This has consequences onto the disposal of coloured objects. A limited number of colouring bodies can be stabilized in the form of inclosure pigments (see EP-A 74.779). The manufacturing process for this kind of pigment is very complicated and expensive.
Coloured pigments based on metal oxide-coated platelet-like substrates, wherein the coating contains at least 10 wt. % Al.sub.2 O.sub.3 are also known (DE-A 4,007,037). Al.sub.2 O.sub.3 reduces the colour strength and gloss of the coloured pigment. The pigment not being coloured in depth, in case of a chemical attack the coloured surface layer is preferably dissolved.
Also the stabilization of colouring bodies in molten corundum is known (JA 78/128600). Owing to the production in the electric arc oven at high temperature, only a very limited colour spectrum can be produced. The manufacture--especially the micronization--of this product is complicated and expensive.
Up to now, it was however not possible to provide a wide palette of colours for incorporation in aggressive mediums, such as enamels, without resorting to heavy metal-containing pigments.
The task was therefore to provide chemically resistant, free-flowing colouring body having a high colour strength and brightness containing no questionable heavy metals.
This task could surprisingly be accomplished by means of the coloured granulates according to the invention.